Added .CodeMirror to end of class names so that under CodeMirror v3, the
authors intended BG/FG is used rather than the parent styling seen

This will make fixedGutter work properly, and make it the only model
(no more gutter that scrolls out of view).

Also reorganizes the basic css file to be easier to modify for users.

Issue #730

It's tiny anyway, no need to force people to include yet another file.

Signed-off-by: Jan Keromnes <janx@linux.com>